1. Field of the Invention
This invention relates to the design and fabrication of integrated circuit devices and, more particularly, to the design of a low-voltage reference generation circuit that provides low reference voltage with a controllable thermal coefficient.
2. Description of the Related Art
As is well known, bandgap voltage reference circuits are commonly deployed in the design of integrated circuit devices. The advantages associated with bandgap voltage reference circuits largely derive from the fact that such circuits are capable of providing a thermally stable voltage reference. In practice, the thermal coefficient of the voltage reference ideally approaches zero. An analysis of a number of embodiments of bandgap voltage reference circuits may be found in the textbook xe2x80x9cAnalog Integrated Circuit Designxe2x80x9d, by David A. Jones and Ken Martin (John Wiley and Sons), pp. 353-364, which is hereby incorporated by reference.
FIG. 1 depicts a bandgap voltage reference circuit that is considered to be representative of the state of the prior art. As may be readily observed, the bandgap voltage reference circuit depicted in FIG. 1 is realized through bipolar junction transistor technology, although other semiconductor device technologies, including MOS, may also be deployed. A realization of the invention based on MOS technology is described in detail in the Description below.
Referring now to FIG. 1, bipolar implementation of a bandgap voltage reference circuit is seen to include a current source Io that is coupled between a voltage source Vs and the emitter of a pnp transistor Q44. Q44 is coupled in a common-collector configuration between Io and GND. The voltage reference also includes npn transistors Q41, Q42 and Q43, each of which has a collector coupled through a respective resistor, R42, R43 or R44, to the emitter of Q44 and to current source Io. The emitters of Q41 and Q43 are directly connected to GND, while Q42 emitter is coupled to GND through resistor R41. The base electrodes of Q41 and Q42 are commonly connected Q41 collector. Q42 collector is in turn connected to Q43 base, and Q43 collector is connected to Q44 base. The output voltage, Vout, of the bandgap voltage reference circuit appears at the interconnection of Io and Q44 emitter.
In order to apprehend the operation of the bandgap voltage reference circuit of FIG. 1, assume for pedagogical purposes that the emitter area of Q42 is an order of magnitude (ten times) greater than the emitter area of Q41. Based on that assumption, an analysis of the operation of the bandgap reference circuit proceeds as follows. The base-to-emitter voltage of Q41, VBE(Q41), is identical to the voltage at Q41 collector. At room temperature, approximately 300xc2x0 K., this voltage is roughly 700 mV. In addition, as may be readily understood from FIG. 1, the voltage at Q42 collector is equal to VBE(Q43). Consequently, the voltages across R42 and R43 are substantially equal. Therefore, if the resistance of R42 is designed to be equal to the resistance of R43, then the currents respectively flowing through these resistors must likewise be equal. As a result, the currents respectively flowing across Q41 and Q42 must be very nearly identical. From the above, and recalling that the emitter area of Q42 is an order of magnitude greater than the emitter area of Q41, it follows that:
I(Q41)=IseqVBE(Q41)/kT=I(Q42)=10IseqVBE(Q42)/kT,
where I(Q41) is the current in Q41, and I(Q42) is the current in Q42.
In the above equation, Is is understood to be reverse saturation current at a specified temperature. It is well known that the reverse saturation current of a bipolar transistor is proportional to its base-to-emitter junction area. Because Q41 and Q42 are fabricated on the same die, according to the same process, and the base-to-emitter junction area of Q42 is ten times that of Q41, the reverse saturation current of Q42 is ten times greater than the reverse saturation current of Q41. Also, in the above equation:
K is Boltgman""s constant,
q is the charge of an electron, and
T is the absolute temperature.
Therefore, xcex94VBE=VBE(Q41)xe2x88x92VBE(Q42)=(kT/q)ln 10.
At room temperature, xcex94VBE is equal to 60 mV and has a positive temperature coefficient of 0.2 mV/xc2x0C. However, from inspection of FIG. 1, it is seen that xcex94VBE is precisely the voltage across R41. If R43=10R41, then the voltage across R43 is 600 mV, with a temperature coefficient of 2 mV/xc2x0C. If VBE (Q43), the base-to-emitter voltage of Q43, has a magnitude of 700 mV, with a temperature coefficient of xe2x88x922 mV/xc2x0C., then the reference voltage, Vout, will have a magnitude of 1300 mV with a zero temperature coefficient.
Accordingly, the prior art provides a technique for synthesizing a temperature-independent voltage reference that, as might be expected, has widespread utility in integrated circuit design. Additionally, the voltage reference is largely insensitive to semiconductor processing variations. However, the bandgap voltage reference circuit that is described above imposes an inherent design constraint that has become increasingly less tolerable as system designs have evolved. That is, because present designs develop a voltage reference, Vout, that is approximately 1300 mV, the voltage source, Vs, must be comfortably greater than 1300 mV in order to drive current source Io. Although prior-art integrated circuit design and fabrication techniques have enabled operation from voltage sources as low as 1.5V, state-of-the-art designs are expected to be driven by power consumption and dissipation considerations to voltage sources as low as 1.2V, or even 1.0V. Clearly, what is required in order to operate from voltage sources as low as 1.2V, is a bandgap reference circuit design that generates a reference voltage much lower than the 1300 mV typically encountered.
The above and other objects, advantages and capabilities are achieved in one aspect of the invention by a circuit that generates a reference voltage having a magnitude less than the generally known silicon bandgap voltage. The circuit includes an amplifier having differential first and second inputs. Three current sources have control terminals coupled to the amplifier output and provide currents of equal magnitudes. The output of the first current source is connected to a first input of the amplifier, and is also coupled through a first junction device to GND. The output of the second current source is connected to a second input of the amplifier and is coupled through a second junction device and a resistance to GND. A third junction device is coupled between the output of a biasing device and GND. A voltage divider is coupled across the third junction device and has an output coupled to the output of the third current source.
Another aspect of the invention is manifest in a circuit for generating a voltage that is less than the semiconductor bandgap voltage. The circuit comprises voltage differential means, a feedback amplifier, first and second current sources, a voltage reference and a resistance element. The voltage differential means develops a voltage differential characterized by a temperature coefficient of a first polarity. A feedback amplifier has an input coupled to the voltage differential means. The first current source has a control terminal coupled to the output of the feedback amplifier and an output coupled to the voltage differential means. A voltage reference develops a voltage having a thermal coefficient of a second polarity, opposite to the first polarity. The second current source is also coupled at a control terminal to he output of the feedback amplifier, and has an output coupled to the voltage reference. The second current source provides a current in proportion to the voltage differential. The resistance element is coupled between the output of the second current source and the voltage reference so that a voltage is developed across the resistance element that is proportional to the current provided by the second current source. The voltage generated by the voltage generation circuit represents the sum of the voltage developed across the resistance element.
In a further aspect of the invention, a voltage generation circuit for generating an output voltage that is less than the semiconductor bandgap voltage comprises a differential amplifier having a noninverting input, an inverting input, and an output. A first semiconductor junction device is coupled between the inverting input of differential amplifier and GND, and a first current source has an output coupled to the inverting input of the differential amplifier and the first semiconductor junction device. A series -connected second semiconductor junction device and a first resistor are coupled between the noninverting input and GND. A second current source has an output coupled to the noninverting input and to the series-connected second semiconductor junction device and first resistor and GND. A voltage reference circuit establishes a voltage reference and equivalent series resistance. The voltage reference circuit comprises a third semiconductor junction device and a resistive divider coupled in parallel with that device. A third current source is coupled to the resistive divider so that the output voltage of the voltage generator circuit consists essentially of the sum of the voltage reference and the voltage across the equivalent series resistance.
In addition, the invention comprehends a method of generating an output voltage that is appreciably lower than the nominal silicon bandgap voltage, which is understood to be approximately 1300 mV. According to the method, a first current is provided to a first semiconductor junction device; and a second current, having a magnitude substantially equal to the magnitude of the first current, is provided to a series-connected second semiconductor junction device and first resistance. The second semiconductor junction device has a junction area that is greater (in a preferred embodiment, by approximately an order of magnitude) than the junction area of the first semiconductor junction device, so that the density of the current flowing through the first junction is proportionately greater than the density of the current flowing through the second semiconductor junction device. The first semiconductor junction device is coupled to the inverting input of a differential feedback amplifier; and the series-connected second semiconductor junction device and resistance are coupled to the noninverting input of the differential feedback amplifier. As a result, the voltage drop across the first semiconductor junction device is greater than the voltage drop across the second semiconductor junction device, and a voltage differential is developed across the first resistance. The magnitude of the second current is proportional to the voltage differential and has a temperature coefficient of a first polarity. A reference voltage is developed that is equivalent to a voltage source in series with the equivalent resistance formed by the parallel equivalent of two resistive elements. A third current, having a magnitude equal to the magnitude of the second current, is forced to flow through the equivalent resistance so that the voltage across the equivalent resistance is added to the reference voltage, thereby creating the output voltage. Because the temperature coefficient of the reference voltage has a polarity opposite the polarity of the temperature coefficient of the second current, the output voltage can be made to have a positive, negative, or zero temperature coefficient simply by selecting appropriate values for resistive elements.